Department of Horticulture
Addressing winter stress and preventing cold damage is essential for strawberry production in the Midwest. Repeated freezing and thawing events which are very common and becoming even more common in the central United States are extremely detrimental to strawberry health. Strawberry buds will perish below 20°F, which can mean no fruit for June-bearing cultivars. Besides no fruit, the roots and crowns can be severely damaged from repeated exposure to negative degree temperatures resulting in plant death. This article will briefly cover some key information on how to successfully overwinter strawberries.
When to cover strawberry patches?
The best time to cover a strawberry patch is after one or two light frosts. This ensures that the plants have properly hardened off before being covered. If done too early the plants may actually be more susceptible to winter injury due to the sudden change in temperature being more drastic in that covered environment. For a time comparison, early to mid-November is standard for covering strawberries in the Midwest.
There are many cover material options!
For traditional mulches, the best material is whatever can be found that has no pesticide residue, is weed free, and large particle sized. Excellent mulching materials include wheat, oat, or soybean straw. Chopped cornstalks are another possibility If they were not treated with herbicides. strawberries are sensitive to many herbicides that are used for burning down of row crops be careful on choosing a source, sometimes the cheapest option isn't the best one. The goal is to apply at least three to five inches of material with the goal of having two to three inch thick mulch after settling.
Depending on the size of the patch, cardboard can be used between the mulch and the plants for an extra layer of protection, for our region it should not be used as the sole source of winter protection. Fallen leaves are not a good winter mulch for strawberries for several reasons but mainly they can mat together in layers, trapping air and creating space for ice to form. The leaf, air, and ice layers do not provide adequate protection and may damage plants due to excess moisture trapped under the material.
Plastic covers work well and are becoming more popular due to ease of placement and removal. Use a thicker plastic and make sure the edges are weighed or tied down to keep heat in. Also in windy conditions sealing the edges is essential for keeping the plastic in place. Many Eastern strawberry protection systems use a combination of plastic and overhead irrigation for additional cold protection by flooding the top of the plastic.
When to remove the covering?
In the spring, the best time to remove any cover option is after the threat of frost is over. After winter, the flower buds are sensitive to temperatures below 32°F or lower. The first flower buds on June bearing cultivars typically have the biggest sized fruit. A good rule of the thumb is to start checking the plants beginning of April and look for 25% of the plants start breaking dormancy. The new growth may not be green (which is fine just alarming) and more yellow or white. Mulch can be reused for weed control in between rows.
For more information on growing strawberries at home in general, please visit: https://store.extension.iastate.edu/product/4057
Currently at Iowa State, the perennial fruit program is planting a few different strawberry cultivars at different timings throughout the fall to observe cold damage and the ability of the plants to overwinter successfully. Their goal is to provide more insight into Iowa fall strawberry plantings in the near future!